An efficient synthesis of [1-C-13]-para-xylene (1a) and [2-C-13]-para-xylene (1b) is described. The incorporation of the label has been achieved by cyclocondensation of suitable 1,5-bis(bromomagnesio)alkanes with either ethyl [1-C-13]acetate or ethyl [C-13]formate which gave [ring-C-13]-labelled dimethylcyclohexanols. Dehydration of these alcohols followed by dehydrogenation of the intermediate dimethylcyclohexenes furnished the title compounds in 32 and 40% overall yield, respectively. Copyright (C) 2002 John Wiley Sons, Ltd.